A piston ring is attached to a piston ring groove formed in an outer periphery of a piston of an internal combustion engine. A piston ring for use in a typical gasoline internal combustion engine has a configuration including two compression rings (top ring and second ring) and a combination oil ring. The combination oil ring has an oil-control function, and is configured to suppress consumption of engine oil.
In general, for the purpose of reducing an oil consumption, in a combination oil ring including two segments (side rails) and an expander spacer (spacer expander), there has been employed a general method of increasing an oil ring tension, or a method of increasing a drop (distance μm in a radial direction), without increasing the oil ring tension, from a vertex SP of an outer peripheral surface S (hereinafter referred to as “outer periphery vertex”) in a certain-length measurement-width so as to maintain a narrow contact width between an outer peripheral slide surface of the segment and a bore in an axial direction in an outer peripheral shape of the segment. Generally, the oil-control performance of the oil ring is indicated by a surface pressure (JIS B8032-13) which is calculated based on an oil ring tension (tangential tension) and a width h1 of the segment. However, the width of the segment is herein replaced with the contact width between the outer peripheral slide surface of the segment and the bore in the axial direction, and the surface pressure is hereinafter referred to as “actual surface pressure”. The actual surface pressure is determined by following equation (1) described below, where a bore diameter is represented by d (mm), an oil ring tension is represented by Ft (N), a contact width between the outer peripheral slide surface of the segment and the bore in the axial direction is represented by h2 (mm), and the actual surface pressure is represented by P (N/mm2).P=Ft/(d·h2)  (1)
When the contact width h2 of the segment with the bore at the time of actual sliding is small, the actual surface pressure P increases in Equation (1) described above, thereby being capable of reducing the oil consumption. Further, increase in oil ring tension Ft causes increase in actual surface pressure P according to Equation (1) described above, thereby being capable of reducing the oil consumption. However, the use of means for reducing the oil consumption may induce increase in friction.
There has been known the following related-art documents relating to such oil ring. In Patent Literature 1, there are disclosed the following inventions of an oil scraper ring. With regard to two discs (side rails or segments) of an oil ring, the two discs are the same, and each have a running surface (distal end outer peripheral surface of the segment) having an asymmetrical convex shape. In one case, vertex lines of the outer peripheral surfaces of the two discs are oriented toward a center of a ring groove. In another case, the vertex lines of the outer peripheral surfaces of the two discs are oriented toward side surfaces opposite to a piston top of the ring groove. Further, a cross section of the running surface of the disc has an asymmetrical shape of a polynomial of the second order expressed by h(x)=ax+bx2 in a first section (I), and after passing a supporting vertex (II) h(x=0) configured as an edge, an asymmetrical shape of the function h(x)=cx2 in a third section (III), thereby improving an oil-scraping action.
In Patent Literature 2, it is disclosed that a rail outer peripheral surface of an oil ring having a substantially I-shaped cross section, which is formed by coupling upper and lower rails to each other with a web, has an asymmetrical barrel-curved surface with a vertex formed at a lower part in the axial direction from a width center in the rail axial direction, and a friction loss force by a single-cylinder motoring tester is reduced.